The potential of lactic acid as a commodity chemical, for example for use in the production of various industrial polymers, is known. The use of lactic acid as a commodity chemical is particularly of interest because polylactic acid and many of its products are biodegradable. Additionally, lactic acid can be produced by fermentation, using renewable carbon sources.
Low pH fermentation is a process under investigation for improving the commercial viability of lactic acid production. In contrast to neutral pH fermentation (e.g., pH in the range of 5.0 to 8.0, inclusive, more typically 5.0 to 7.0, inclusive) in which the lactate material is mainly present as a lactate salt, lactate material formed using a low pH fermentation (e.g., at a pH below 5.0, typically below 4.8, more typically below 4.3) includes a significant amount of the free acid form. Thus, isolating lactic acid from an aqueous solution having a pH below 5.0 tends to reduce the need for acidulation technology typically necessary for isolating lactic acid from an aqueous solution having a pH above 5.0.
Many acidification technologies have significant capital costs or operating costs; consume acidification reagents; and/or result in the consumption of reagents and formation of by-product salts. Therefore, there is an interest in developing organisms that are capable of retaining high productivity at pH ranges of 5.0 or lower.
In addition to the effort to develop organisms having high productivities at a low pH, there is an effort to increase the use of mixed sugar streams during fermentation. Unlike traditional fermentation media, which typically includes a purified sugar stream such as dextrose as a carbon source, mixed sugar streams include combinations of hexoses, hexuloses, and pentoses such as dextrose, galactose, fructose, arabinose, and xylose. Mixed sugar streams are a lower cost carbon source that can be obtained, for example, by enzymatic or acid hydrolysis of cellulose and hemicellulose. U.S. Pat. Nos. 5,562,777, 5,620,877 and 4,350,766 discuss methods for making mixed sugar streams. U.S. Pat. Nos. 5,798,237 and 5,789,210 discuss use of these streams as a carbon source for fermentation. The disclosure of these five patents are incorporated by reference herein.
Although mixed sugar streams provide a relatively low cost carbon source, mixed sugar streams typically have more impurities, such as lid than conventional dextrose streams. Not only do the impurities increase the burden on the microorganism, which must tolerate the impurities, the impurities also must be separated from the fermentation broth and the lactate material. Therefore, a separation process tat is capable of reducing the presence of impurities due to the use of mixed sugar streams, or other carbohydrates sources, is also desirable.